Taylor vortices formed in a thin membrane channel are known to prevent membrane fouling and to increase flux by reducing the boundary layer near the membrane surface due to enhanced mass transfer of solute or suspended materials from the membrane surface to the bulk of the solution.
Conventional membrane devices known to increase flux are spinning discs, spinning annular cylinders, and vibrating discs.
Some examples of spinning disc membrane devices are set forth in U.S. Pat. No. 4,925,557 (Ahlberg, Jr. et al.), which issued May 15, 1990, U.S. Pat. No. 4,025,425 (Croopnick et al.), which issued May 24, 1977, U.S. Pat. No. 4,132,649 (Croopnick et al.), which issued Jan. 2, 1979, and U.S. Pat. No. 4,576,715 (Michaels et al.), which issued Mar. 18, 1986. The Ahlberg, Jr. et al. patent discloses a rotating membrane filter unit wherein the central portion of hollow filter discs covered with controlled pore size membranes communicate with the interior of the hollow rotating mounting shaft with the membrane surface of the rotating disc being kept clear by rotation and, in some cases intermittent back flushing, and with filtrate draw off thru the hollow rotating mounting shaft.
U.S. Pat. Nos. 4,025,425, 4,132,649 and 4,576,715 all relate to a purifying apparatus which includes a stack of filtration packs, each including a sheet-like carrier and a pair of membranes lying over opposite faces of the carrier. The packs are assembled along a predetermined axis into a stack, with radially inner and outer gaskets separating adjacent packs from one another. The inner gaskets are stacked to form the equivalent of a hollow shaft. The assembled stack of packs and gaskets forms a rotor which is rapidly rotated, while water or other feed fluid is fed along the shaft formed by the inner gaskets and flows through grooves in the gaskets into the space between adjacent membrane packs. As the rotor rotates, the feed fluid flows radially outwardly, so that pure water or other permeate contained in the feed fluid, permeate through the membranes and passes radially outwardly along the pack to the periphery, where the permeate then flies off into a collecting vessel.
Spinning annular cylinder membrane devices are set forth in U.S. Pat. No. 3,669,879 (Berriman), which issued on Jun. 13, 1972, and 3,567,030 (Loeffler), which issued on Mar. 2, 1971. The Berriman patent discloses a separation process of the reverse osmosis type useful for generating fresh water by forcing salt water against a semipermeable membrane that allows only pure water to pass, which uses centrifugal forces created by rotating a basket to build up a pressure head of water supplied to the basket to force water through the membrane disposed about the basket. Similarly, the Loeffler patent relates to a separation device for separating materials by the process of reverse osmosis wherein centrifugal force is used to carry out the reverse osmosis.
The present inventor has discovered that a enhanced membrane device can be created by combining the rotating features of the above-mentioned patents and a spiral wound membrane. The novel spiral wound rotating membrane device provides a design which increases flux and reduces fouling by a combination of shear forces, centrifugal forces and Taylor vortices.
Spiral wound membranes have been used in liquid/solid separation as evidenced by U.S. Pat. No. 3,872,014 (Schell), which issued on Mar. 18, 1975. However, these conventional spiral wound membrane devices are typically stationary in design.
Although U.S. Pat. No. 3,669,879 (Berriman) discloses a spiral wound membrane disposed about the outer basket of a rotating membrane device, it does not disclose the unique configuration of the rotating spiral wound membrane according to the present invention so as to produce a membrane device which provides increased flux by the combination of shear forces, centrifugal forces and Taylor vortices, i.e., a whirl or eddy. That is, the spiral wound membrane applied according to the Berriman patent about the outer basket only increases the flux rate by means of the centrifugal forces generated by the rotation of the basket, whereas the design of the present invention increases flux not only by centrifugal force, but also via the Taylor vortices and shear forces created by its unique placement about the center feed shaft. By rotating the novel spiral wound membrane around its axis at a speed from 10 rpm to 20,000 rpm the present inventor believes that the rotation will form the above-mentioned flow effect which will increase flux rates and reduce fouling.
The present invention also provides many additional advantages which shall become apparent as described below.